The present invention relates to a solid polymer electrolyte fuel cell.
A solid polymer electrolyte fuel cell has a structure comprising a gas diffusion electrode provided on both surfaces of an ion exchange membrane (solid polymer electrolyte). This is an apparatus which allows the electrochemical reaction of an oxidizing agent such as oxygen with a fuel such as hydrogen to give electric power.
A gas diffusion electrode comprises a catalyst layer and a gas diffusion layer. The catalyst layer is formed by binding a catalyst such as particulate noble metal and/or carbon powder having a particulate noble metal supported thereon with a binder or the like. As such a binder a fluorinic resin such as polytetrafluoroethylene (PTFE) is normally used. This fluorinic resin is also a water repellent which renders a catalyst layer properly water-repellent. As a gas diffusion layer a water repellent carbon paper or the like is used.
The characteristics of such a solid polymer electrolyte fuel cell drastically depend on the structure of the gas diffusion electrode, particularly the catalyst layer. In other words, the electrode reaction proceeds on a three-phase boundary where the catalyst in the catalyst layer, the electrolyte and oxygen or hydrogen are present. However, since this type of a fuel cell comprises a solid material as the electrolyte, this three-phase boundary is limited to a two-dimensional boundary of electrolyte with catalyst layer, lowering the activity of the gas diffusion electrode. Attempts have been made so far to enhance the activity of gas diffusion electrodes by various methods for increasing the three-phase boundary.
One of these methods is to increase the surface of solid polymer electrolyte membrane and hence its area of contact with a catalyst. For example, JP-A-58-7423 (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) proposes a process for the production of a porous polymer electrolyte membrane. However, the above cited patent application has no reference to effects and characteristics developed when this production method is applied to fuel cell. Further, JP-A-4-169069 proposes a method involving the roughening of the surface of a solid polymer electrolyte membrane by sputtering or like means.
Another method is to incorporate an ion exchange resin in a catalyst layer and hence increase its area of contact with a catalyst. For example, JP-B-62-61118 (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) and JP-B-62-61119 propose a method which comprises the preparation of a catalyst layer from a mixture obtained by adding an ion exchange resin solution to a catalytic substance. JP-A-4-162365 employs a method involving the coating of the surface of a catalytic substance by an ion exchange resin solution. JP-B-2-48632 and JP-A-6-333574 propose a method which comprises spraying or applying an ion exchange resin solution to a catalyst layer, and then drying the catalyst layer to provide the catalyst layer with an ion exchange resin. Further, JP-A-7-183035 proposes a method which allows a catalytic substance to adsorb an ion exchange resin colloid.
On the other hand, another factor affecting the characteristics of a solid polymer electrolyte fuel cell is the electrical conductance of the solid polymer electrolyte membrane. In other words, in order to enhance the power of a solid polymer electrolyte fuel cell, it is important to lower the resistance of solid polymer electrolyte membrane. To this end, a method involving the provision of a solid polymer electrolyte membrane having a reduced thickness or a method involving the increase of the amount of sulfonic group incorporated in ion exchange resin has been proposed.
Although the foregoing conventional methods can increase the surface area of the ion exchange resin membrane in the catalyst layer itself, it can hardly fill the pores in the catalyst layer or the valleys of unevenness on the catalyst layer with a catalytic substance such as particulate carbon having a particulate noble metal catalyst supported thereon. Thus, it is extremely difficult to increase the three-phase boundary by these methods.
Further, a method is disclosed which comprises covering a catalytic substance such as particulate carbon having a particulate noble metal catalyst supported thereon to form a catalyst layer having an increased contact area and hence an increased three-phase boundary as mentioned above. In this case, it is indispensable to leave the catalytic substance partly uncovered by using a water repellent such as PTFE to improve the properties of fuel cell or form a uniform thin coating film to enhance the gas permeability thereof. If a portion left uncovered is formed, a portion which is excessively covered or entirely uncovered due to the positional relationship between catalytic substances is formed, lowering the gas permeability or making it impossible to obtain the desired activity. Accordingly, the resulting fuel cell exhibits deteriorated properties. Further, if the method involving the formation of a uniform thin coating film is employed, it is extremely difficult to form such a uniform thin coating film, deteriorating the productivity. In addition, if the thickness of the coating film is reduced, the path of migration of proton is remarkably reduced, deteriorating the properties of the fuel cell.
In addition, if the thickness of the coating film is reduced, the path of migration of proton is remarkably reduced, deteriorating the properties of the fuel cell.
Therefore, the present invention has been worked out to give a solution to the foregoing prior art problems. An object of the present invention is to provide a gas diffusion electrode and a solid polymer electrolyte which increase the three-phase boundary in the catalyst layer while sufficiently securing the path of migration of substances such as oxygen, hydrogen and produced water all over the catalyst layer and the catalytic substance without deteriorating the ionic conductivity thereof in a solid polymer electrolyte fuel cell. Another object of this invention is a high power solid polymer electrolyte fuel cell comprising the gas diffusion electrode and solid polymer electrolyte. Another object of the present invention is to provide a process for the production of a gas diffusion electrode which can also secure electrical contact with a catalytic substance.
The foregoing objects of the present invention are accomplished by the following inventions.
The first invention concerns a gas diffusion electrode for a solid polymer electrolyte fuel cell comprising a gas diffusion layer and a catalyst layer, characterized in that said catalyst layer is provided with a catalytic substance and an ion exchange resin having pores.
The second invention concerns a gas diffusion electrode for a solid polymer electrolyte fuel cell comprising a gas diffusion layer and a catalyst layer, characterized in that said catalyst layer is provided with an ion exchange resin having pores having a diameter of from 0.05 to 5.0 xcexcm and porosity of not less than 40%.
The third invention concerns a gas diffusion electrode for solid polymer electrolyte fuel cell comprising a gas diffusion layer and a catalyst layer, characterized in that said ion exchange resin is a perfluorosulfonic acid resin and said catalytic substance is or comprises a particulate noble metal or carbon having a particulate noble metal supported thereon.
The fourth invention, which is according to the first, second or third invention, concerns a process comprising forming an ion exchange resin coating film, made of a solution obtained by dissolving an ion exchange resin in a solvent containing an alcohol, on a catalyst layer precursor with the ion exchange resin coating film on it in an organic solvent having a polar group other than alcoholic hydroxyl group so that the ion exchange resin is solidified and rendered porous. In the present invention, the coating film may be in the form of a membrane or may comprise a catalytic substance incorporated in an ion exchange resin. To be short, it suffices if the ion exchange resin is present around the catalytic substance.
The fifth invention concerns a solid polymer electrolyte membrane-gas diffusion electrode assembly comprising a gas diffusion electrode for a solid polymer electrolyte fuel cell according to any one of the first to third inventions provided on at least one side of a solid polymer electrolyte membrane.
The sixth invention concerns a solid polymer electrolyte fuel cell comprising a solid polymer electrolyte membrane-gas diffusion electrode assembly according to the fifth invention.
The seventh invention concerns a solid polymer electrolyte membrane comprising an ion exchange resin as a constituent and having pores.
The eighth invention concerns a solid polymer electrolyte membrane according to the seventh invention, wherein said solid polymer electrolyte membrane has a pore diameter of from 0.02 to 1.0 xcexcm and a porosity of not less than 10%.
The ninth invention concerns a solid polymer electrolyte membrane according to the seventh or eighth invention, wherein said ion exchange resin is a perfluorosulfonic acid resin.
The tenth invention, which is according to the seventh, eighth or ninth invention, concerns a process for the production of a solid polymer electrolyte membrane which comprises soaking a solution comprising an ion exchange resin dissolved in a solvent containing an alcohol in an organic solvent having a polar group other than an alcoholic hydroxyl group so that said ion exchange resin is solidified and rendered porous to form an ion exchange resin membrane having pores.
The eleventh invention concerns a solid polymer electrolyte fuel cell comprising a solid polymer electrolyte membrane according to the seventh, eighth or ninth invention.